Real-time location (RTL) systems have become popular in recent years. Conventional RTL systems typically use radio frequency (RF) transmission to determine location. The RF based methods typically do not provide enough accuracy to support room and sub-room level accuracies, because the RF transmissions may penetrate walls. The use of infrared (IR) has become more popular for some RTL systems in order to support room level accuracy, because infrared signals (and any other optical wavelength) may be completely blocked by walls. Accordingly, for IR based RTL systems, if an infrared signal is received within an enclosed room, it may be assumed that a portable device, that may transmit or receive the infrared signal, is within the room.
The conventional “IR method” typically employs an IR transmitter on a portable device (i.e. a tag) and IR receivers in base stations that are scattered in rooms and corridors within the enclosure. A portable device ID is typically received by one of IR the base stations and the location of the portable device is determined based on its vicinity to a base station. A portable device-based IR transmitter typically needs a line of sight between the transmitter and a receiver (i.e. a base station) in order to robustly detect the device ID. This type of RTL system, thus, is occasionally prone to a lack of reception by the receiver base stations when the line of sight is blocked, making it susceptible to reliability problems.
There is also another type of IR system that employs IR transmitters at the base stations and IR receivers at the portable device which attempts to solve the problem of poor IR sensitivity, by transmitting substantially higher power levels than wired stationary base stations are typically capable of.